George Zimmerman wasn’t charged with murder last week; “neighborhood-watch captain” George Zimmerman was. Whatever the outcome of his trial, Zimmerman has given a decidedly bad name to the neighborhood watch — enough so that 20th Century Fox decided to pull the trailer for a comedy of that name.

The image of Zimmerman as a gun-toting vigilante is a violent twist on the suburban busybody stereotype that the Fox movie plays with — and, indeed, a severe departure from the unarmed norm. But the flaw in neighborhood watches isn’t that they can tempt a “captain” to abuse make-believe authority. It’s that they embody and can promote a narrow, fear-driven approach to citizenship.

Local anticrime watches emerged in the U.S. during the social turmoil of the late ’60s. In 1972, the National Sheriffs’ Association created the National Neighborhood Watch Program to standardize a process for organizing and operating them. Today there are over 25,000 such patrols, varying in size and formality. Although watches have generally been found to reduce crime, much of the academic research on their effectiveness was done in the ’90s, when, for a variety of reasons, crime rates began falling nationwide.

So in a sense they are a success. But most neighborhood watches are founded out of reaction and most stay there. They leave us defining civic life as looking out for danger, citizenship as making citizen’s arrests. Gated communities, like the one in Sanford, Fla., where Trayvon Martin was killed, are often civically barren deserts. They exude an ethic of gated citizenship, a mind-set of suspicion and threat.

Banding together to protect against threats is perhaps the most primal kind of “civic engagement.” Human evolution, however, has meant the emergence of forms of cooperation that are affirmative and constructive rather than merely defensive. Societies that thrive figure out how not only to guard but also to build.

Over the past several decades, as Richard Sennett points out in his new book, Together: The Rituals, Pleasures and Politics of Cooperation, those deep habits of constructive citizenship have fallen away in the U.S. More of us live alone than ever before. Fewer of us join, let alone lead, civic groups or voluntary associations. Trust and social capital are in decline, which makes people even less likely to join and participate. So what was the most primal form of citizenship is now the only remaining one in many communities. We watch for strangers who don’t belong. We forget how to make them belong.

Imagine, instead, a neighborhood watch oriented not toward catching criminals or perceiving them in unfamiliar kids but toward simply buildingneighborhood. Creating trust by showing courtesy. Cleaning up empty lots. Helping those who’ve suffered a loss. Planting a pea patch together. Crafting ways for the oldest to mentor the youngest. It’s a safe bet that this approach would be at least as effective at preventing crime.

Fortunately, neighborhood watches have exactly the right scale and structure for this affirmative work. The anthropologist Robin Dunbar has observed that throughout human history, the maximum size of a coherent community, from Neolithic farming settlements to companies of Roman soldiers, is about 150. What if we converted neighborhood watches into Dunbar units, companies of citizens who want to cultivate trust and relationships and social capital? A neighborhood or a housing project of 1,000 units is not really a neighborhood. A neighborhood of 10 sets of 100 homes, each connected to the others — that’s more like it. In this kind of network, small acts of citizenship can compound.

To be sure, there are many neighborhood watches in America that have made such a pivot: from street patrols to street parties, from roadblocks to roadside gardens. They are not the norm, though. There are other neighborhoods, including in my city of Seattle, where people already live with a strong spirit of community. But they aren’t always organized, via any kind of Dunbar structure, to give sustained and replicable form to this spirit.

Martin’s death was caused as much by an underdeveloped community as an overeager watchman. The fate of the watchman is in the hands of the law. The health of the community — be it Sanford, Seattle or any American town — is in our own hands. If we want to live in civic gardens rather than deserts, we’d better learn to do more than just watch.